16.2 Soil–plant nutrient relations
Soil–plant interrelations are dynamic and subject to both inputs (fertilisers, pollutants, soil chemistry) and losses (erosion, leaching, harvesting). Metal ions are released into soil solution via weathering and solubilisation of soil minerals (Section 16.1) as well as via decomposition of organic matter.
Available ions are those that a particular root system can acquire. Strictly speaking, only ions in soil solution would be considered available, but due to a dynamic equilibrium that exists between the soil solution and other ion pools from which ready transfer into the soil solution can occur, ions adsorbed onto exchange sites can also be considered as available, or at least as influencing the available fraction.
Factors affecting ion supply to plant roots include ion activities in the soil solution, usually referred to as intensity, and the degree and rate of replenishment of the soil solution ion pool from other pools (ions adsorbed on solid soil particles or labile organic compounds and ions present in other readily soluble compounds), usually referred to as capacity. The capacity factor therefore determines a buffer power for a particular metal. The relationship between capacity and intensity factors for each particular metal is heavily dependent on pH.
(a) Cation exchange capacity
Clay particles are negatively charged and therefore surrounded by a swarm of (positively charged) cations. Clay minerals owe a part of their negative charge to isomorphous substitution (cations of higher charge, like Al3+, are replaced by those of lower charge, like Mg2+) thus leaving a surplus of (non-
neutralised) negative charges which are satisﬁed with adsorbed, exchangeable cations. In addition, soil colloids (e.g. humus, hydrous oxides) that exhibit protonated complexing functional groups (–OH, –COOH) also contribute to the cation exchange capacity of soils:
Since dissociation of –OH and –COOH groups (especially those on organic matter) is pH dependent, cation exchange capacity increases with an increase in pH. With an increased cation exchange capacity, metal cations are attracted to these negative sites on solid particles, soil solution is depleted and therefore metal availability reduced.
(b) Retention of cations in soils
Cations are held more strongly (less reversibly) when pH increases from 5 to 7. Cu, Zn, Ni, Cd and other metals become signiﬁcantly less soluble and less exchangeable when pH increases from 5 to 7. Retention of metals in soil can occur through several processes: (1) cation exchange (non-speciﬁc adsorption), (2) speciﬁc adsorption, (3) organic complexation and (4) co-precipitation. In a given situation most, if not all, of these processes contribute to metal retention in soils.
In order to maintain electroneutrality, negative charges on solid particles (soil colloids) are balanced by an equal amount of cations; an exchange refers to the exchange between counter-ions balancing the surface negative charge on the soil colloids and ions in soil solution. Such an exchange is reversible, stoichiometric and diffusion controlled. Moreover, there is a certain degree of selectivity of the adsorbent. The higher the valency of an ion, the greater its replacing power (H+ behaves like a polyvalent cation). By contrast, with greater degrees of hydration, a given ion will exhibit a lower replacing power.
Adsorption by cation exchange represents electrostatic binding through the formation of outer-sphere complexes with the surface functional groups. An outer-sphere complex means that at least one molecule of a solvent comes between the functional group and the ion.
Speciﬁc adsorption is pH dependent and related to the hydrolysis of the heavy-metal ion. In speciﬁc adsorption partly covalent bonds are formed with the lattice ions. Partly covalent bonds are inherently stronger than electrostatic binding involved in the non-speciﬁc cation exchange (e.g. Zn can be adsorbed on Fe and Al oxides 7 and 26 times more strongly than their corresponding cation exchange capacity at pH 7.6 would imply). Metals most able to form hydroxy complexes are speciﬁcally adsorbed to the greatest extent:
Hg > Pb > Cu >> Zn > Co > Ni > Cd
Speciﬁc adsorption may also include diffusion of metals into mineral interlayer spaces and their ﬁxation there. Such diffusion increases with an increase in pH.
Organic matter may either increase or decrease availabil-ity of micronutrients, Al and heavy metals. Reduced availability is due to complexation with humic acids, lignin and other organic compounds of high molecular weight (insoluble precipitates are thus formed). Conversely, increased availability may result from solubilisation and thus mobilisation of metals by low molecular weight organic ligands (e.g. short-chain organic acids, amino acids and other organic compounds). Stability constants of chelates with several metals occur with increasing order as:
Cu > Fe = Al > Mn = Co > Zn
Co-precipitation represents formation of mixed solids by simultaneous precipitation as occurs with Fe and Mn oxides.
(c) Soil–plant pH
The pH value most relevant to soil and plant chemical processes is pH of the soil solution. A soil is acidic if the pH of its aqueous solution phase is <7 and alkaline if that pH exceeds 7. Nutrient element availability varies accordingly (Figure 16.2), and beyond the range of pH 4–8 plant growth becomes a function of pH per se, plus pH effects on nutrient ion availability.
In chemical terms, pH represents a measure of H+ activity in a soil solution which is in a dynamic equilibrium with a negatively charged solid phase. H+ ions are strongly attracted to these negative sites and have sufﬁcient power to replace other cations from them. A diffuse layer in the vicinity of a negatively charged surface has higher H+ activity than the bulk soil solution.
Soil pH varies in time and space. Diurnal fluctuations of as much as one pH unit may occur, as well as spatial variations (horizontal and vertical down the soil proﬁle). Soil pH also varies over seasons. During seasons with low to moderate rainfall when evapotranspiration greatly exceeds precipitation, salts are not being removed by deep percolation and increased salts tend to reduce pH by forcing more of the exchangeable H+ ions into the soil solution. Conversely, during wet seasons, salts are removed from the topsoil and pH goes up. This season to season fluctuation in total salt content should not be confused with long-term soil acidiﬁcation (Section 16.5).
(d) Relationships between pH and ion toxicity
Soil pH is a dominant influence on solubility and therefore availability and potential phytotoxicity of metals (Figure 16.2). Low pH favours free metal cations and protonated anions, higher pH favours carbonate or hydroxyl complexes. There-fore, availability of micronutrient and toxic ions (which are present in soil solution as cations) increases with increasing soil acidity. By contrast, availability of those present as anions (MoO42–, CrO42–, SeO4–, SeO3– and B(OH)4–) increases with increasing alkalinity (see Case study 16.1).
Plant growth is dependent on availability of water and nutrients in the rhizosphere, the soil–root interface consisting of a soil layer varying in thickness between 0.1 mm and up to a few millimetres depending on the length of root hairs (Section 3.3). Availability of nutrients in the rhizosphere is controlled by the combined effects of soil properties and interactions between plant roots and adjacent microorganisms in the surrounding soil.
Chemical conditions in the rhizosphere are usually very much different from those in the bulk soil further away from roots. Root-induced changes in the rhizosphere pH are a result of the balance between H+ and HCO3– excretion, evolution of CO2 by respiration and loss of various organic compounds known collectively as root exudates.
The balance between H+ and HCO3– excretion depends upon the cation/anion uptake ratio. Greater excretion of H+ accompanies a greater absorption of cations than anions and results in rhizosphere acidiﬁcation. Conversely, when uptake of anions exceeds uptake of cations, excretion of HCO3– exceeds that of H+. The chemical form of soil N (ammonium v. nitrate) is an influential factor for the cation/anion ratio. Ammonium-fed plants take up more cations than anions, and they usually have a more acidic rhizosphere than bulk soil, while nitrate-fed plants take up more anions than cations and show the opposite relationship between rhizo-sphere and bulk soil pH. Plant effects on rhizosphere pH also vary with genotype, which can in turn influence nutrient ion availability (Section 3.3.1).
Overall, plants and soils must be regarded as interacting components in any ecosystem, and because plants take up more basic than acidic components, any net increase in ecosystem biomass will result in some degree of soil acidiﬁcation.